Use of Steam Tables and Mollier Chart

Definition

Steam tables and the Mollier chart are essential thermodynamic tools used by engineers to determine the properties of water and steam (such as pressure, temperature, enthalpy, entropy, and specific volume) at various states, allowing for the precise analysis of power plants and thermal systems.

Main Content

1. Steam Tables

• Steam tables provide tabulated data for water and steam in different states: compressed liquid, saturated liquid, wet steam, and superheated steam.
• These tables are categorized by either temperature (where pressure is the variable) or pressure (where temperature is the variable) to help find properties quickly.

2. The Mollier Chart (h-s Diagram)

• The Mollier chart is a graphical representation of the properties of steam, plotting Enthalpy ($h$) on the Y-axis and Entropy ($s$) on the X-axis.
• It is particularly useful for visualizing adiabatic processes, such as the expansion of steam through a turbine, where the process appears as a nearly vertical line.

3. Understanding State Regions

• The chart and tables define three primary regions: the subcooled region (left of the saturated liquid line), the wet region (inside the "dome"), and the superheated region (right of the saturated vapor line).
• The "dome" represents the transition phase where liquid and vapor coexist; the quality ($x$) of steam indicates how much of the mixture is vapor.

       Enthalpy (h)
           |      / (Superheated Region)
           |     / 
           |    /   
           |  _/____ Saturated Vapor Line
           | /  |
  Wet      |/   |
  Region   |    |
___________|____|_______ Entropy (s)
    Saturated Liquid Line

Working / Process

1. Identify the State of Steam

• First, determine if the steam is saturated (at boiling point), wet (a mixture), or superheated (above saturation temperature).
• Use the given pressure and temperature to compare against the saturation values provided in the tables.

2. Locate Data in Steam Tables

• If the state is known, look up the corresponding table (e.g., Saturated Steam Table).
• For wet steam, use the dryness fraction ($x$) and the formula $h = h_f + x(h_g - h_f)$ to calculate specific enthalpy.

3. Plotting on Mollier Chart

• Find the starting point by locating the intersection of given Enthalpy and Entropy coordinates.
• Follow the process line (e.g., constant entropy for an isentropic process) to reach the final state point and read the new property values directly from the axes.

Advantages / Applications

• Power Plant Efficiency: Steam tables and Mollier charts are vital for calculating the Rankine cycle efficiency in thermal power stations.
• Turbine Analysis: Engineers use the Mollier chart to determine the work output of a steam turbine by finding the enthalpy drop during expansion.
• System Design: They facilitate the sizing of boilers, condensers, and piping systems by providing accurate data on steam density and heat energy content.

Summary

Steam tables and the Mollier chart are indispensable thermodynamic references used to calculate state properties of water and steam. While tables offer precise numerical accuracy, the Mollier chart provides a visual overview of energy changes during thermodynamic processes.

• Key terms to remember:
• Enthalpy ($h$): The total heat content.
• Entropy ($s$): A measure of thermal energy unavailability.
• Dryness Fraction ($x$): The ratio of the mass of vapor to the total mass of the mixture.
• Saturation: The state where liquid and vapor coexist at a specific pressure.